In recent years, activity in solar cell research has increased considerably. This research is driven by the need for solar-to-electricity conversion devices having increased efficiency. To meet the demand for increased efficiency, a small but rapidly growing segment of photovoltaic technology research has focused on so-called second generation solar cells. These devices include thin films fabricated with materials such as Cu(InxGa1-x)Se2 (CIGS) and CdTe, and the like. Thin-film-based photovoltaic technologies offer potential long-term high-efficiency, and represent an economically-viable alternative for large-scale solar energy conversion and power generation. These technologies provide better solar energy conversion at lower cost than established silicon (Si) technologies, but their efficiency needs to be enhanced for making them practically viable.
The development of thin-film based solar conversion devices has been hampered by low film quality, complex manufacturing processes, and low scale-up yield. In an effort to address these problems, third generation photovoltaic devices, including multi-junction cells, dye sensitized solar cells, bulk hetero-junction devices, and organic cells, including tandem cells, have been developed to combine the advantages of both the first and second generation devices. These devices, however, exhibit either high manufacturing costs, or poor conversion efficiency, usually of less than about 10%, or poor durability and photo stability.
One of the central issues leading to low conversion efficiency in conventional planar p-n junction devices and, in particular, thin-film hetero-junction devices, relates to a limited minority carrier diffusion length as compared to the thickness of the absorber film. This problem results in the inevitable charge recombination due to the existence of a large concentration of defects, such as dislocations, vacancies, and impurities.
In an effort to improve the charge collection efficiency and for further cost reductions, nano-structured materials, such as nanorods, nanowires, and nanotubes have been explored as components for photovoltaic devices. Compared to thin films, nano-scale materials enable substantial light absorption due to a large surface area and good light trapping properties. Devices employing nano-structured materials also benefit from short range, efficient spatial carrier separation, which alleviates one of the key problems of the various forms of planar device architectures. In effect, a semiconductor device consisting of nano-structures arranged in vertically-aligned arrays of radial p-n junctions can more effectively suppress the non-radiative, bulk recombination events and, hence, may relax the stringent requirements on material quality. Despite their high efficiency potential, however, to date fabricated photovoltaic solar cells based on such nano-structured assemblies yield conversion efficiencies of only about 0.5 to 3%. This is far below the computer simulated, theoretical efficiency limits of around 11%.
The low conversion efficiency of these vertically-aligned, p-n junction devices is typically caused by non-optimized dimensions, low cell density, poor cell alignment, or lack of proper ordering of the nano-structures. Further problems relate to low p-n junction interface quality. Furthermore, the fabrication of these nanostructures generally requires use of complicated fabrication techniques or multiple-step processing schemes that are unsuitable for large-scale manufacturing. To date, controlled and cost-effective fabrication of large-area, nano-structured assemblies for photovoltaic devices have yet to be demonstrated.